Variations of the concentration of dimethyl sulfur compounds (DMSCs) and the effects of environmental factors on the concentrations of DMSCs were investigated in a seawater mesocosm experiment from June to July 2005. Three large algal blooms, an unknown algal bloom, a diatom bloom, and a dinoflagellate bloom, respectively, were developed in the experimental enclosures. Three peak concentrations of DMSCs, corresponding to three algal blooms, indicated that algal blooms played an important role in the production of DMSCs. This study also showed that dinoflagellate bloom contributed the most to DMSCs, unknown algal bloom the least, and diatom bloom was intermediate. PO3-4, NO-2 and NH+4affected the productions of dimethylsulphoniopropionate (DMSP) and dimethylsulfoxide (DMSO) through the influence on the growth of algae, and NO-2 and NH+4might even affect the productions of DMSP and DMSO through the control of their physiological roles in the algal cell. However, no significant effects of PO3-4, NO-2 and NH+4on the production of dimethylsulfide (DMS) were evident.